1. Field of the Invention
The present invention generally relates to computerized image reconstruction and, more particularly, to image reconstruction from projected data along curved paths of proton particles through an object being imaged.
2. Description of the Related Art
Computed tomographic image reconstruction has typically focused on X-rays or photons that pass through the body in a straight line. Any path deviation from an original direction is considered as the path of scattered particles and may be ignored. Such imaging modalities, such as X-ray Computed Tomography (xCT) or simple CT, Single-Photon Emission Computed Tomography (SPECT) and Positron Emission Tomography (PET), provide high energy X-rays or Gamma-rays that travel along a straight line path inside the body. The various image reconstruction methodologies and algorithms developed and applied to these imaging modalites generate satisfactory image quality for diagnosis and other medical applications.
However, travel paths of heavy particles, such as proton, muon, neutron, carbon, alpha particle, do not follow a straight line within the body, and present image reconstruction difficulties for proton Computed Tomography (pCT).
Clinical applications of pCT and proton beam treatments in medicine provide improved radiation therapy due to the distinct advantages of proton beams compared to other radiation therapy options, such as X-rays, electron beams. Proton beams deliver radiation energy in a precise manner and leave normal tissues around targeted diseased tissue, such as a tumor, mostly unharmed or undamaged. Thus, there is a growing demand for efficient, low-cost and more accurate proton beam radiation therapy, which has sparked great scientific interest in the development of image reconstruction from projected data along proton paths that may not necessarily be straight lines through the object to be imaged.
Proton therapy for cancer treatment has gained more recognition and attracted great interest in the past decade as evidenced by the increased number of proton radiation therapy facilities. Different from an X-ray beam, which losses intensity by photon absorption and scatter along the entire beam path of a straight line/strip, a proton loses energy through elastic and inelastic collisions with atomic electrons and nuclei, resulting in a nonlinear path of traversal. This mode of interaction also results in an energy deposition phenomenon known as the Bragg peak in which the proton releases a burst of energy around the end of its trajectory. By this property, protons have the potential to deliver a desired radiation dose to a targeted cancerous volume with minimal harm to the normal tissues.
However, existing proton treatments have several problems associated with radiation dose calculations due to varied positioning of patient anatomy, and are currently performed based on X-ray computed tomography or xCT, with the patient positioned with the help of X-ray radiographs, hence direct visualization of the three-dimensional (3D) patient anatomy in the treatment room is presently impossible, limiting the accuracy of proton therapy. Further, conventional practices for estimating the proton path rely on single line-type curve estimations and fail to consider proton deflection and scattering within the body.
Conventional proton therapy utilizes X-ray beams to produce the computed tomographic images (i.e., xCT) or the X-ray attenuation map of the body for treatment planning. Because of the difference of X-ray and proton interaction with the body tissues, the xCT attenuation map introduces an uncertainty into proton therapy treatment planning. Since the Bragg peak releases a large amount of energy locally, a minor error on the targeted cancerous volume could cause significant harm to the surrounding normal tissues.
Thus, the accuracy of xCT for proton treatment planning is limited due to the difference in physical interactions between X-ray photons and particle protons, which partially obviates the advantage of proton therapy. Further, conventional methods do not accurately modify xCT in the treatment room based on patient position.
Thus, there is a need for more accurate image reconstruction of proton beams to provide improved proton dose distributions and verification of the patient position on the treatment table, and to develop accurate 3D imaging techniques.